Methods for measuring instantaneous crop yield as a grain harvester is moving through a field of grain have been the subject of prior inventions. Methods for measuring the mass flow rate of grain in a harvester have been used to provide a grain flow rate signal which can be used to calculate either the total weight of grain harvested within a given field area or the instantaneous yield of the crop at the present location of the harvester in the field. These data are useful to the agricultural producer to allow measuring the effect of different soil conditions or crop growing practices on crop yield. Total weight of grain is calculated by integrating grain mass flow rate versus time. Instantaneous crop yield is calculated by dividing instantaneous grain mass flow rate by the instantaneous rate at which the harvester is harvesting the field area.
Numerous methods have been used to measure mass flow rate of grain on harvesters. U.S. patent application Ser. No. 07/716,293, now U.S. Pat. No. 5,343,761, discloses means and methods for measuring grain mass flow rate at the exit of a paddle type chain conveyor, by measuring the force resulting from grain striking an impact plate as grain exits the conveyor. Other methods, such as measuring electrical properties, such as capacitance, of grain passing by or contacting a means for sensing said properties, have also been used.
All of the known means for measuring mass flow rate use a sensor which provides an electrical output signal which varies in a linear or non-linear relationship with mass flow rate. Accurate calculation of mass flow rate from this signal requires that the value of this signal be accurately known at both the mass flow rate to be measured and at a baseline condition of zero mass flow rate. The difference in the output signal of the sensor between these two conditions can be used in combination with a calibration characteristic to estimate actual mass flow rate. With many of the known sensors and their associated electronic signal conditioning, changes in the baseline output signal occur due to many causes; such as drift in electronic signal conditioning, thermal or mechanically induced stresses in the sensor means, and variation in baseline sensor output at zero mass flow rate due to unwanted but unavoidable changes in operating conditions. An example of the latter type of real but unwanted sensor signal is a change in the baseline output signal of an impact force measuring sensor, as shown in said application, due to varying inclination of a harvester as it operates within a crop field with varying ground slope.
Slow changes in the baseline output signal of a sensor at zero level of the measured parameter is often termed zero or baseline drift, and is a widely recognized problem with many types of sensors. The usual method for minimizing errors caused by baseline drift is to build the sensor and associated signal conditioning with very high accuracy, so that most of the causes of drift are minimized in magnitude. This is usually costly, and in some cases, impossible to do at any cost.
Another approach to minimizing errors caused by baseline drift is to use a method for cancelling or correcting for drift by periodically measuring the output signal of the sensor when the measured parameter is known to be at a zero level. U.S. Pat. Nos. 3,714,806 and 3,791,204 show methods for holding the output signal of force indicating means on steel rolling machines at zero when no steel is passing between the rollers of the machines, which is a known condition of zero rolling force. This method functions well for a steel rolling application, because rolling force is the only significant force which can be applied to the force sensor, and essentially all other changes in indicated force are due to baseline drift. However, this method does not function well for an impact type grain mass flow sensor in a harvester, because variation in operating slope of the harvester can cause continual variation in the baseline output signal of the flow sensor, due to the weight of the sensing mechanism, while the sensor operates continuously at a non-zero mass flow rate condition. A zero mass flow rate condition does not exist with sufficient frequency to allow this method to be used successfully on a harvester.
U.S. Pat. Nos. 3,434,062 and 3,359,410 show methods of correcting for baseline drift of the output signal of a chromatograph. These methods utilize the fact that the output signal characteristic of a chromatograph consists of a slowly drifting baseline value which is periodically interrupted by sharp peaks representing the quantities being measured. Baseline drift correction is applied when the output signal is changing slowly, but is stopped when the signal is changing rapidly. This method of drift correction does not function well for an impact type grain mass flow sensor in a harvester, because the slopes of the baseline and the useful output signals from an impact sensor are not as clearly differentiated as with a chromatograph. For example, significant peaks and valleys may exist in the baseline signal due to mechanical vibration of the impact sensor due to operation of the harvester.